Transmembrane receptors are proteins which are localized in the plasma membrane of eukaryotic cells. These receptors have an extracellular domain, a transmembrane domain and an intracellular domain. Transmembrane receptors mediate molecular signaling functions by, for example, binding specifically with an external signaling molecule (referred to as a ligand) which activates the receptor. Activation results typically in the triggering of an intracellular catalytic function which is carried out by, or mediated through, the intracellular domain of the transmembrane receptor.
There are various families of transmembrane receptors that show overall similarity in sequence. The highest conservation of sequence is in the intracellular catalytic domain. Characteristic amino acid position can be used to define classes of receptors or to distinguish related family members. Sequences are much more divergent in the extracellular domain.
A variety of methods have been developed for the identification and isolation of transmembrane receptors. This is frequently a straightforward matter since receptors often share a common sequence in their catalytic domain. However, the identification of the ligands which bind to, and activate, the transmembrane receptors is a much more difficult undertaking. Brute force approaches for the identification of ligands for known receptors are rarely successful. Brute force approaches usually depend on a biological activity that can be monitored (e.g., nerve growth for nerve growth factor; or glucose homeostasis for the insulin receptor) or they depend on finding a source of the ligand and using affinity to purify it (as was used to find the c-Kit ligand in mouse hemopoietic cells). In general, however, a source of the ligand is not known, nor is there an obvious or easily assayable biological activity. Therefore, there are many receptors, referred to as "orphan receptors" for which no corresponding ligand has been identified. A systematic approach to the identification of receptor ligands would be of great value for the identification of ligands having useful pharmacological activities.